A laser beam may be emitted while it is focused into collimated light. Therefore, the laser beam can perform scanning two-dimensionally to display an image using a mirror element. This type of a scanning-type laser projector is power saving, as this projector displays an image by modulating an intensity of the laser beam, when compared to a common projector that always illuminates an entire two-dimensional image display device, such as a liquid crystal panel or a digital micromirror device (DMD). In addition, the device may have a small size as the device does not require an illumination optical system that evenly illuminates the two-dimensional image device. In particular, the small device may employ a MEMS mirror element capable of performing oscillation control of a mirror having a diameter of about 1 mm at a frequency of several ten kilohertz.
Thus, the scanning-type laser projector can be mounted onto a small mobile device so as to allow enjoyment of a large-screen display even with a mobile phone on which it is difficult to mount a large display.
On the other hand, safety standards determine permissive radiation power for laser application products, such as laser projectors, and a technique for making the display bright by increasing radiation power while ensuring safety is demanded. In particular, with the scanning image display device, it is predicated that scanning of the laser beam does not stop, as an intensity of a laser beam that enters human eyes as pulsed light by scanning is calculated, and the permissible exposure level is calculated based on a safety level of the intensity.
Therefore, it is necessary to reduce a possibility that a laser beam of a danger level is radiated outside the device as much as possible, considering various failure modes. In general, a sensor monitors an operation of a scanning mirror, and the laser is turned on after it is confirmed that scanning is normally performed. When amplitude of the scan is below amplitude within a range of the emitted laser beam or stops, an output of the laser is suppressed or the laser is stopped. However, it is desired to address the above problem, as the laser control system may go out of order or in failure from some reasons.
For example, in a conventional scanning image display device described in PTL 1, a scanning mirror is previously pulled by a spring. By driving the scanning mirror with a force over a force of the spring, the mirror moves to a position at which the laser beam is not irradiated when driving of the mirror stops.
Further, in a conventional scanning image display device described in PTL 2, generation of power supplied to a laser light source stops when an abnormality occurs in scanning.
Further, in a conventional scanning image display device described in PTL 3, when an output of a laser abnormally increases, an optical element converts its characteristic and optical transmittance or reflectance decreases.
Each of the patent literatures describe prevents of a laser beam that is not scanned from being emitted due to some causes of a failure, malfunction of laser control in a worst case. However, with the above conventional image display devices, it is difficult to prevent a laser beam that is not scanned from being emitted due to a combination of various reasons of a failure, malfunction of laser control in a worst case.